This　paper　focused　on　the　major　particle-size　based　foulants　responsible　for　membrane　fouling　of　ultraflltration　in　simulated　surface　water　polluted　by　sewage　discharge.　The　experiment　was　conducted　with　a　sequential　filtration　process,　pre-filtration　steps　removed　specific　foulants　(1.2　mu　m,　0.45　mu　m,　100　kDa　and　10　kDa)　from　raw　water　with　different　membranes,　and　followed　filtration　tests　were　performed　to　investigate　the　fouling　caused　by　remaining　foulants.　Flux　decline　and　specific　fouling　resistance　of　different　fractioned　substances　were　compared　to　identify　the　particle-size　based　foulants　most　responsible　for　membrane　fouling.　The　normalized　flux　decline　showed　that　foulants　over　1.2　mu　m　had　limited　influence　on　flux　decline,　but　that　the　100　kDa-0.45　mu　m　fraction　contributed　a　significant　part.　Results　of　specific　fouling　resistance　revealed　that　the　100　kDa-0.45　mu　m　fraction　caused　higher　fouling　resistance　than　other　fractions　with　the　same　quality　of　organic　matter,　and　this　fraction　was　confirmed　to　be　most　closely　related　to　protein-like　substances.　Component　analysis　demonstrated　that　protein-like　substances　played　a　crucial　role　in　membrane　fouling,　whereas　humic-like　substances　were　of　minor　relevance.　Standard　blocking　and　cake　filtration　were　major　mechanisms　of　membrane　fouling　caused　by　pre-filtered　water,　independent　of　the　pore　size　of　fractionation　membranes.　(C)　2014　Elsevier　B.V.　All　rights　reserved.